support Return None and fall-off

Author: AndrewTolmach

type_check_Lfun.py

@@ -81,6 +81,8 @@ def type_check_exp(self, e, env):
           case _:
             raise Exception('type_check_exp: in call, unexpected ' + \
                             repr(func_t))
+      case Constant(None):
+        return VoidType()
       case _:
         return super().type_check_exp(e, env)
 

type_check_Lgrad.py

@@ -58,6 +58,8 @@ def type_check_exp(self, e, env):
     match e:
       case Constant(value) if value is True or value is False:
         return BoolType()
+      case Constant(None):
+        return VoidType()
       # Cases for Lvar
       case Name(id):
         return env[id]